Lock simplecov to last known 1.8-compatible version

Simplecov 0.9 is not 1.8-compatible anymore due to `Dir.home`:
colszowka/simplecov@34696d40b782fa2a9fb8fc2bb7d0bb3281a134cf

https://travis-ci.org/lostisland/faraday/jobs/35757490

I have no idea how this previous build passed, however:
https://travis-ci.org/lostisland/faraday/jobs/34619493